I caught up to Bishop following the taping of a KET show "What's The Big Idea?" that will televised December 22 (watch it), and asked him to describe the nub of the idea behind the book. He obliged:

Whereas economic opportunity might have been the primary reason to relocate in years past, similar migrations occur today because people also seek the comfort of like-minded people. Comfort may indeed be found, but it comes at the cost of bewilderment: "How can they believe that?!" When even the facts are up for grabs, incredulity prevails.

Richard Florida, who collaborated on the book with Bishop, approaches touches on this sorting out in his "creative class" work as well.

And finally, the word of the day: "The Big Sort" is also another way of saying "homophily," which Ethan Zuckerman writes about here, and talks about, here.